The long term goal of this research project is to elucidate the immunological mechanisms by which NK cells regulate T cell responses during infections with intracellular bacteria. Human monocytotropic ehrlichiosis (HME) is a highly prevalent life-threatening tick-borne disease in North America caused by E. chaffeensis. The infection can be subclinical or fatal, mimicking toxic shock syndrome. Although management of this disease relies on doxycycline, there is a high rate of treatment failure when the drug is administered late in infection. Our extensive murine studies indicated that fatal Ehrlichia-induced shock is due to CD8+ T cell-mediated immunopathology. IFN-gamma producing-CD4+ Th1 cells are critical for the elimination of Ehrlichia; however, these cells undergo suppression and late apoptosis during fatal disease. We recently demonstrated that NK cells impair effective bacterial elimination and mediate tissue injury during fatal Ehrlichia-induced shock. However, the immunoregulatory mechanisms that account for the pathogenic role of NK cells in Ehrlichia- induced shock remain unknown. We hypothesize that NK cells negatively regulate the generation of protective anti-erhrlichial immune responses by suppressing CD4+ Th1 cell responses, while promoting the induction of pathogenic CD8+ T cells and tissue injury during fatal Ehrlichia-induced toxic shock. We will test this hypothesis using three specific aims. In Aim 1, we will determine the impact of NK cells on the functions of protective CD4+ Th1 cells during fatal ehrlichial infection. In Aim 2, we will identify the molecular mechanisms by which NK cells contribute to the pathogenesis of Ehrlichia-induced toxic shock. In Aim 3, we will characterize the NK cell responses throughout the course of Ehrlichia infection in humans. We will carry out these aims by: 1) measuring the differences in expansion and effector functions of CD4+ T cells and CD8+ T cells in Ehrlichia- infected NK-depleted mice or sham controls; 2) comparing the proliferation and cytokine production of effector CD4+T cells when co-cultured in vitro with NK cells derived from lethally- or non-lethally infected mice in the presence or absence of Ehrlichia-infected antigen presenting cells (APCs); 3) comparing CD4+ T cell and CD8+ T cell responses in irradiated bone marrow mouse chimeras, in which IL-10 will be only expressed by non-hematopoietic cells or non B/T lymphocyte-innate hematopoietic cells including NK cells and APCs; and 4) comparing NK and T cell responses in patients with severe and mild ehrlichiosis and determine whether in vitro depletion of human NK cells alters the proliferation and functions of human CD4+T cells in these patients. HME is not only an emerging public health concern, but represents a model for toxic or septic shock caused by other bacteria. Therefore, knowledge generated herein will facilitate rational development of novel therapeutics against infection-induced sepsis.